Figure 1-8. Topographic map of Mauna Loa and Kilauea, Hawaii
(b) Drainage Developed on Extrusive Rocks. In areas of volcanic cones, streams generally radiate
from the center of the cone, forming a radial drainage pattern. On flat-lying lava flows, drainage may range from
dendritic to indiscernible due to internal drainage through characteristic columnar jointing. Large-scale parallel
drainage frequently occurs in areas where planar flow surfaces are slightly inclined. The formation of gullies
varies widely depending on the origin and manner of emplacement of the extrusive igneous rock formation.
Gentle, U-shaped gullies will form on well-developed cohesive soils, while V-shaped gullies form on more
granular material, such as tuff.
(c) Vegetation in Areas of Extrusive Rocks. Young volcanic cones that have been recently active
or have steep side slop are unlikely to develop much vegetation. Likewise, young lava flows located in arid or
semiarid environments are usually barren due to a thin soil cover. However, in humid or tropical climates,
extrusive igneous rock flows weather rapidly, forming a thick, clay-rich soil profile in a short period of time.
Consequently, deciduous vegetation is a predominant feature in such areas.
(4) Engineering Properties of Extrusive Igneous Rocks. The engineering properties of extrusive
igneous rocks vary widely. Basalt is a hard, tough, durable source of aggregate for macadam, paving, and
concrete, but it is seldom used as a building stone because of its dark color. Felsite is also an excellent aggregate,
except when used with portland cement. (Silica present in the felsite chemically reacts with the alkalis of portland
cement to create a gel that will absorb water, causing the cement to expand and crack.) The engineering properties
of pumice and scoria are not as desirable as those of basalt and felsite. Neither